Supercritical pressure boiler



Feb. 20, 1968 c. R. MIDTLYNG 3,369,526

SUPERCRITICAL PRESSURE BOILER Filed Feb. 14, 1966 H) IO 22 F/RsT "1 SECOND PRIMARY sEcolvoARY BOILE BOILER suPER- suPER- SECTION SECTION HEATER HEATER FLASH TANK DE-A EREA T0R EE NDENSER MAIN FEED PUMP v CARL R. MIDTLYNG' 36 I Y INVENTOR.

3,369,526 SUPERQRITHCAL 'PREfiS'URE BUTLER Carl R. Miritlyng, Worcester, Mass, assi nor to Riley Stoker Corporation, Worcester, Mass, a corporation of lviassachusetts Filed Feb. 14, .1966, Ser. No. 527,298 Claims. (Cl. 122-406) ABSTRACT 05 THE DECLQSURE This invention relates to a supercritical pressure boiler and, more particularly, to a forced flow boiler having two boiler sections with differing fiow resistance and recirculating means to selectively bring about various fiow rates in the two boiler sections at various loads.

In the design of supercritical pressure boilers it has been common practice to recirculate the efiiuent from the furnace wall, from its outlet to its inlet, employing pumps to accomplish this function. Due to the large difference in specific volume of cold ambient-temperature water which the pump delivers at the beginning of the cold start-up of the boiler and of the hot furnace effluent of the normally operating boiler which the pump delivers in the hot running condition, a pump sized for the hot running condition develops a pressure and capacity on cold start-up which is quite excessive as compared with the actual cold start-up flow requirements in the furnace wall tubes. This is particularly true for the case in which the pump is located in a parallel location. Even if one resorts to the use of two small pumps with only one of them used for the beginning of the cold start-up to reduce the problem to within more acceptable limits, this still leaves excess pressure and capacity at the beginning of cold start-up. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.

It is, therefore, an outstanding object of the invention to provide a supercritical pressure boiler in which a recirculating system is used more effectively in its full capacity during cold start-up as well as during normal operation.

Another object of this invention is the provision of a supercritical pressure boiler having a two'section boiler system in which one section has substantially less resistance to fluid flow than another.

A further object of the present invention is the provision of a supercritical pressure boiler having a novel recirculation system providing a different nature of re circulation during cold start-up and during hot clean-up as compared with normal operation.

It is another object of the instant invention to provide a supercritical pressure boiler in which the furnace wall tubes are subjected to recirculation in a different manner than the convection pass tubes.

With these and other objects in view, as will be apparent to those skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.

The character of the invention, however, may be best understood by reference to one of its structural forms, as illustrated by the accompanying drawings in which:

FIG. 1 is a schematic view of a supercritical pressure boiler embodying the principles of the present invention, and

FIG. 2 is a vertical sectional view somewhat schematically outlined of a furnace and boiler system in accordance with the invention.

If the recirculating circuit of a supercritical pressure boiler includes only the furnace peripheral wall portion 3,3695% Patented Feb. 20, 1968 of the through-flow circuit, the hot clean-up phase of cold start subjects only the furnace peripheral walls to a high flow rate. This is the high flow rate which is necessary to detach and hold in suspension the oxide deposits on the tube walls while the fluid temperature is passing through the 500 F. or 600 F. range. Under these conditions and with this arrangement, the convection pass walls downstream of the furnace peripheral walls (in the through-flow sense) are subjected to a low throughflow rate only. To detach and hold in suspension the oxides on the surfaces of the tubes in the convection pass walls during hot clean-up requires higher flow rates than the startup through-flow only. In the use of the prior art arrangements during hot clean-up, not only will convection tube oxide deposits be inadequately detached but oxides from the furnace walls may re-deposit in the convection pass wall headers and later carry over into the superheaters and turbine. Since there is excess pump capacity with the parallel pump arrangement during hot cleanup, this capability can be employed to flush the convection pass walls by providing alternate connections from the pump suction side to both the furnace exit and the convection pass exit. One valve would be open and the other valve closed during the beginning of cold startup untilsome time after hot clean-up is finished; thereafter, the alternate situation would prevail. Not only will this arrangement presumably subject the convection pass wall tubes to adequate hot clean-up but it will also permit the use of larger tubes in the convection pass and, hence, a lower through-flow pressure drop. This is done by maintaining an adequate cooling flow rate during the time during start-up when through-flow only would not be adequate for cooling, unless small diameter and large through-flow rate pressure drop tubes were employed.

Referring to FIGURE 1 of the drawings, which best shows the general features of the invention, it can be seen that the supercritical pressure boiler, indicated generally by the reference numeral 10, is provided with a first boiler section 11 and a second boiler section 12. A main feed pump 13 causes fluid to pass seriatim through the first boiler section and the second boiler. section. A recirculating means, such as a pump 14, is connected upstream of the first boiler section, The pump is connected by a first line 15 and by a second line 16 to points immediately downstream of the first and second boiler sections, respectively. A valve 17 is located in the first line 15 and a valve 18 is located in the line 16, valve 18 being of the flow-check variety permitting flow only toward the pump 14-. Between the main feed pump 13 and the first boiler section 11 is located an economizer 19. Following the second boiler section 12 is a primary superheater 21 connected to a secondary superheater 22 which, in turn, is joined to the high-pressure section of a steam turbine 23. The high-pressure section of the turbine 23 exhausts into a reheater 24 which, in turn, is connected to the low-pressure section of the turbine. The low-pressure section discharges into a condenser 25 which, in turn, discharges to a demineralizer 26 from which fluid passes through feed heaters (not shown) and a deaereator 27 which, in turn, are connected back to the main feed pump 13.

A suitable flash tank 28 is capable, on occasion, of receiving fluid from between the second boiler section 12 and the primary superheater 21 and delivering it to the discharge of the condenser 25 among other things. On one side of the pump 14 is located an isolating valve 29, while a similar valve 31 is located on the other side of the valve 14. The valve 31 contains a check section permitting flow of fluid only toward the first boiler section.

Referring to FIG. 2, Which shows the application of the system to a steam generating unit, the supercritical pressure boiler is shown in use with a furnace 32 having a combustion chamber 33 which is fired at the bottom by burners 34. The first boiler section 11 is formed as the Walls of the combustion chamber 33 and consists of tubes 35 of relatively small diameter extending between a bottom distributor system, such as a header 36 and an upper collector system, such as a header 37. The secondary superheater 22 is located in the upper part of the combustion chamber33. A horizontal convection.

pass 38 extends horizontally from the upper part of the combustion chamber 33, and in this horizontal convection pass is located the reheater 24 and the primary superheater 21. The rear end of the convection pass 38 is connected to a vertical convection pass 39 at the upper portion of which is located the second boiler section 12. This boiler section is made up of back pass wall tubes 41 which join a lower header 42 to an upper header 43 and which are of relatively large internal diameter and present low resistance to flow. Lying throughout the back pass is the eoonomizer 19 although, for the purposes of clarity of illustration, the economizer is shown entirely below the header 42. The output side of the pump 14 is connected through the valve 31 to a mixing chamber 44 which, in turn, is connected by a line 45 to the lower header 36, the line being divided to provide for flow distribution to points in the header 36. The main feed pump 13 is connected to the inlet of the economizer 19 and the outlet of the economizer is connected to the mixing chamber 44. The central portion of the header system 37 is connected to the lower header 42 of the second boiler section 12, thus providing it with higher than average temperature fluid produced in the furnace. The ends of the upper header 37 are connected through the valve 17 and the valve 29 to the pump 14, thus providing the pump with the lower than average temperature fluid emitting from the first boiler section. The recirculating pump 14 is selected of a size sufficient to provide flow of hot .fiuid through the first line only during normal operation and to provide flow of cold fluid through the second line only during cold start-up.

The operation of the apparatus will now be readily understood in view of the above description. With the burners 34 operating and causing hot gaseous products of combustion to pass upwardly through the combustion chamber 33, horizontally through the convection pass 38, and then vertically downwardly through the back or vertical convection pass 39, all of the heat exchange ele ments in the supercritical pressure boiler are subjected to the action of the hot gas. The main feed pump causes the through-flow to pass first through the first boiler section 11 and then through the second boiler section 12. Fluid is recirculated during normal operation by the pump 14 from the output of the first boiler section, that is to say, through the valve 17 and through the line 15 to the beginning of the first boiler section. During the cold start-up and the hot clean-up phase, however, the pump --14 is used to recirculate fluid from the output of the second boiler section 12, that is to say, through the line 16 and through the valve 18. At that time, the valve 17 is closed and prevents flow of fluid through the line 15. The pump 14 is selected of an adequate head-capacity characteristic to pump suflicient fluid from the line 15 to cause recirculation and to bring about the adequate fluid flow through the first boiler section to prevent damage to the tube, particularly during hot start-up and during normal operation. When this same pump is used during cold start-up and hot clean-up, because of its headcapacity characteristic, there is a large excess of capacity which has no useful purpose in a normal boiler. However, with the present boiler, by closing the valve 17 and opening the valve 18, fluid is caused to recirculate from the output of the second boiler section 12. This provides suflicient flow to protect the tubes in this section during start-up; also, the second boiler section can be pro vided with large internal diameter tubes having low resistance to flow because, at this particular critical time in the operation of the boiler, it is not necessary to provide small diameter tubes to maintain adequate proteoctive how in the second boiler section, as would otherwise be true if the present invention did not produce the extremely high recirculated flow through this second boiler section.

It can be seen, then, that a number of beneficial effects come about by use of the present construction. First of all, in pump 1 which must necessarily be selected large enough to provide for the recirculation of hot fluid through the first boiler section 11 during normal operating, low load, and hot start-up conditions has an excess of pumping capacity when cold fluid is being recirculated during cold start-up and during hot clean-up. This excess capacity is used for providing for rccircuation through an additional section, i.e., the second boiler section 12 during cold start-up and hotclean-up, when such recirculation is beneficial to the second boiler section. The only time the second boiler section is subjected to dangerous gas conditions is during cold start-up and hot clean-up and in :1 conventional boiler of this type whichwould require small tubes to maintain adequate flow rate at that time. Such small tubes are expensive to fabricate begin with and, secondly, present considerable resistance to flow during normal operation, thus requiring a higher pressure main teed pump 13 and associated equipment. In other words, the second boiler section would otherwise contribute substantially to the overall pressure drop through the boiler. With the present invention, the recirculation through the second boiler section 12 takes place only during these critical periods when the second boiler section presents a problem. This permits the designer to use large tubes in the second boiler section, thus having the effect of permitting a reduced design pressure for the main feed 13. As a matter of fact, almost all of the excess recirculating pump capacity is used in this extra recirculation circuit.

It should be noted, of course, that one possibility for removing the problem of excess capacity in the re-,

circulating pump is to use two pumps in parallel and to take one pump off the line during cold start-up and hot clean-up. This solution to the problem is not entirely.

satisfactory because it requires a much larger number of controls, two small pumps are always more expensive than one large one, and, of course, each pump must be isolated :by suitable expensive valves. Furthermore, no matter how these pumps are divided, there is always some excess capacity in the one that is left on the line during the cold start-up.

The problem of oxide deposits is, of course, adequate ly taken care of by the present invention because, during the cold start-up and hot clean-up, the fact that the boiler has been in a cold condition means that oxides have been formed and are released during the rise in temperature of the boiler. If adequate flow does not take place through both sections of the boiler, oxides will be dropped off in the second boiler section. Nevertheless, it is not necessary to have recirculation through the second boiler section because the through-flow is adequate to protect the tubes. Also, one would not want to recirculate through the second boiler section during normal operating, because the hot fluid has a specific gravity which makes it very diflicult to pump and would require an extraordinarily large recirculation pump and because no useful purpose would be served by such operation.

It is obvious that minor changes may be made in the form and construction of the invention without departing from the material spirit thereof. It is not, however, desired to confine the invention to the exact form herein shown and described, but it is desired to include all such as properly come within the scope claimed.

The invention having been thus described, what is claimed as new and desired to secure by Letters Patent is:

1. A supercritical pressure boiler having a furnace and a back pass, comprising a) a first boiler section of relatively small sized tub- (b) a second boiler section provided with tubes of relatively larger internal diameter,

(c) a main feed pump causing fluid to flow seriatim through the first boiler section and the second boiler section,

((1) recirculating means having an outlet connected upstream of the first boiler section and an inlet connected by a first and a second line to points immediately downstream of the first and second boiler sections, respectively, and

(e) valve means located in the first and second lines to permit selective flow of fluid through the lines to the inlet of the recirculating means, the second boiler section having smaller resistance to flow than the first boiler section and smaller resistance than would be used if circulation were not used during cold start-up.

2. A supcrcritical presure boiler as recited in claim 1, wherein the valve means are operated to provide flow of hot fluid through the first line only during normal operation and to provide flow of cold fluid through the second line only during start-up.

3. A supercritical pressure boiler as recited in claim 1, wherein the first boiler section consists of furnace 6 wall tubes and the second boiler section consists of back pass tubes.

4. A supercritical pressure boiler as recited in claim 1, wherein the first boiler section consists of furnace wall tubes of relatively small size, wherein the second boiler section consists of back pass tubes of relatively large size, and wherein the recirculating means includes a single pump which in conjunction with the valve means operates to provide flow of hot fluid through the first line only during normal operation and to provide flow of cold fluid through the second line only during start-up.

5. A supercritical pressure boiler as recited in claim 4, wherein a main feed pump, an economizer, the first boiler section, the second boiler section, a superheater, a turbine, a condenser, and a demineralizer are connected in a series circuit and wherein a flash tank is connected between the outlet of the second boiler section and the outlet of the condenser.

References Cited UNITED STATES PATENTS 3,03 8,453 6/ 1962 Armacost. 3,164,134 1/1965 Kochey. 3,213,835 10/1965 Egglestone 122-406 CHARLES I. MYHRE, Primary Examiner. 

